This section provides background information related to the present disclosure which is not necessarily prior art.
A flyback transformer (FBT) or a line output transformer (LOPT) is a type of transformer which operates at a very high switching frequency and generates high voltage. Unlike a mains transformer, a flyback transformer (FBT) or a line output transformer (LOPT) is designed not just to transfer energy, but also to store it for a significant fraction of the switching period. This is achieved by winding the coils on a ferrite core with an air gap. The air gap increases the reluctance of the magnetic circuit and therefore its capacity to store energy. A flyback transformer generates high voltage in the range of a few kilovolts and operates with switched currents at much higher frequencies in the range of kilo-hertz.
The air gap has a very important role in the design of a transformer. Whenever a gap is inserted in a magnetic path there is a fringing flux induced at the gap, which increases with increase in the size of the air gap. Fringing flux decreases the total reluctance of the magnetic path. If the fringing flux is strong enough, it will induce eddy current both in the core as well as the adjoining winding and will cause localized heating resulting in increased losses and consequently failure of the transformer.
There is a requirement for optimizing the air gap of a flyback transformer or any device with energy storage capability in a ferrite core. Typically, flyback transformers use high permeability ferrite core sections made of individual laminations cut in shapes like ‘E’, ‘I’ and ‘C’ that are bonded together. Prior art in creating a gap for E core geometry and the likes is to grind the inner leg or limb of the ferrite core. Another method is to insert a film at the mating face of the two halves of the ferrite cores on the two outer legs or limbs or bond the core sections using a high temperature masking tape such as kapton tape. One more method known in the art is the use of an iron powder block placed at the mating face of the inner leg or limb of ferrite cores to replace the air gap and minimize the fringing flux.
Grounding of the transformer core serves as a solution for EMI (electromagnetic interference) and prior art in core grounding also has a lot of variations. In mobile and imaging industries, faraday's copper plate shield was used in direct contact with the ferrite core and the lead of the ground pin. Over a period of time, faraday's copper plate shield wrapped around the transformer body evolved to just a small piece of copper plate placed on top of the core surface with the lead terminated on the ground pin. This was further simplified by completely removing the copper plate and using conductive epoxy to provide a coupling between the lead terminal (enamel coating is removed or tinned at the end portion of lead) and the core. According to another method, a tinned wire is wrapped around the transformer body and has direct contact with the core surface. Still another method is to provide a special pin having direct contact with the core surface which proved to be an expensive solution.
Various techniques have been used in the art to provide a low cost solution for energy storage in transformers and cost effective core grounding for reduced EMI.